A Russian expert said on Tuesday a vaccine against swine flu that according to the latest reports has already killed more than 150 people in Mexico could be developed within six months.

The World Health Organization (WHO) has raised its alert level from three to four indicating a significant risk increase and two steps short of a full pandemic.

"A vaccine against swine flu could be developed at best in six months," said Dmitry Lvov, director of the Virology Research Institute at the Russian Academy of Medical Sciences.

WHO officials also said developing a new vaccine could take four to six months.

Lvov assessed the risk of a pandemic spread in the worst scenario as quite high, and warned that swine flu could reach Russia "within hours, or days maximum, anyway within a week."

"It [swine flu] can spread as fast as lightning with the current means of transportation. It is impossible to prevent the virus from entering our country," the Russian expert predicted.

However the director of the State Scientific Virology and Biotechnology Center said on Tuesday the virus currently poses no threat to Russia.

"We do not regard the situation as dangerous in the foreseeable future," Ilya Drozdov said.